Mountain Pass

Mountain Pass is the highest point on I-15 between California and Nevada. The summit sits at 4,730 ft in San Bernardino County, about 15 miles southwest of the state line, where the Nevada side begins at the casino town of Primm. The name does double duty here. It is the literal I-15 summit and also an unincorporated community, home to the Mountain Pass rare-earth mine.

For a trucker, this is a brake grade more than a snow grade, though it is both. Northbound, the freeway drops off the summit into the Ivanpah Valley toward Primm, a downgrade that runs about ten miles before it flattens near Yates Well Road. The grade is roughly 6% for that stretch (Gribblenation, April 2021), which is the best available figure rather than a posted Caltrans spec. A runaway truck ramp is set up for northbound traffic near the dry Ivanpah lakebed.

The pass has no fixed seasonal closure. It is open year-round and shuts only when a winter storm puts snow and ice on the summit, usually for hours, occasionally overnight. When that happens, Caltrans District 8 and the CHP close I-15 between Baker and Primm, then reopen it with escort convoys through the worst of it. Real-time status comes from Caltrans QuickMap or the road hotline at 1-800-427-7623.

• Summit elevation 4,730 ft, the highest point on I-15 between California and Nevada (Wikipedia: Interstate 15 in California)
• Located in San Bernardino County, about 15 miles southwest of the Nevada state line; Primm, NV sits on the Nevada side (Wikipedia: Interstate 15 in California)
• Northbound descent into the Ivanpah Valley lasts about 10 miles down to Yates Well Road, Exit 291 (AARoads: I-15 North, Barstow to Nevada)
• Grade on that descent is roughly 6% (Gribblenation, April 2021); no official Caltrans grade spec was found, so treat this as an attributed estimate
• Runaway truck ramp is provided for northbound traffic near the dry Ivanpah lakebed (AARoads)
• No fixed seasonal closure; the pass closes ad hoc during snow and ice, with documented closures in Dec 2019, Feb 2023, and Feb 2026 (Review-Journal; VVNG; Fox5 Vegas)
• Annual precipitation at the Mountain Pass community averages 8.36 in, wettest in July and August (Western Regional Climate Center, via Wikipedia)

There is no chain-control season on this pass in the way the Sierra passes have one, and no posted closing date. Mountain Pass stays open year-round. California's three-tier chain system can apply in any snow event: R-1 (chains on all vehicles except passenger cars and light trucks under 6,000 lb GVW running snow tires on two drive wheels), R-2 (chains except 4WD under 6,500 lb with snow tires on all four), and R-3 (chains on everything, no exceptions). On this stretch, though, the documented Caltrans and CHP response to ice has been full closure and escort convoys rather than a posted chain-up area. No named chain-up area or chain-control mile marker for Mountain Pass was verifiable. The road closes only when a storm hits and reopens once crews clear the ice, usually within hours. Check Caltrans QuickMap or call 1-800-427-7623 before you run it in winter.

The Barstow-to-Nevada alignment, then US 91/466, was kept as a planned Interstate after the Federal-Aid Highway Act of June 29, 1956. The Mountain Pass segment itself was brought up to freeway standard in October 1963, when the road opened to freeway standards from Cima Road northeast to the Nevada state line. The wider corridor came together in pieces: the 25-mile "Baker Grade" segment from eastern Baker to Halloran Summit was finished in 1961, and I-15 north of Barstow was announced complete to freeway standards in 1965. These 1961, 1963, and 1965 dates trace to Gribblenation, a well-sourced roadgeek site citing period California Highways and Public Works, not a state DOT page directly.

More recently, Caltrans rebuilt I-15 from Devore to Nevada starting in 2002 at a cost of about $349 million, with most of the work done by winter 2009 (Wikipedia: Interstate 15 in California). The Mountain Pass community is now dominated by its rare-earth mine, owned by MP Materials since July 2017, which has produced a large share of the world's rare-earth output.